I have been over at Dudley's pages at Joppa last night looking at his little mini square burners and it started me thinking about building a little demo backyard pot furnace in the 10 to 15 pound or so range and glory to play with in a location where I don't have ready access to a lot of electric.

This lead me to think about burner placements and inside volume for good flame burn.

I've a lot of old books I've picked up over the years, many from the 70's and early 80's and you find a lot of unique ways they use to do things. A lot of the old studio furnaces were top fired t-pipe burners of one sort or the other like Nick Labino did back in the early 60's.

I've always wondered why did he put it in the top of the furnace like that? Was it because he wanted it directed at the melt in order to help it along? Ease of construction? I know it was copied extensively in a lot of the written documentation to follow.

I've done some reading in the archives and I realize you don't want a burner in the top of the furnace due to push back from combustion and heat. You would have to push more propane in order to maintain heat. That's why they started side firing and putting an exhaust vent in. Was the push back coming because it wasn't vented in any way other than the door? I get the part that heat rises and that will make it more likely to cause a pop back as it heats everything up, so putting the burner in the side is the well accepted rule now. I'm curious about things because I've either blown at anothers studio, or nearly a decade ago with my own equipment, but it was a small electric furnace and a small simple fiber glory I built back then. I've never actually built/run a small gas furnace before.

Plus, I've always wondered why Nick Labino chose to start them out in the top like he did. Seems like a lot of extra work to build it that way, but everyone did it that way for a long time.

Pete VanderLaan

11-04-2011, 03:46 PM

In 1962 as a group the movement was all holding hands and no one really knew what they were doing and it never occurred t them to ask in West Virginia. I don't really even think they had burners on the first ones, just straight pipe that was really hard to light.

You will still see an occasional top fire and you can be pretty sure the person who runs it doesn't get out much. I converted all the furnaces for Pilchuck in 1977 from top to side fire. They work far better with flues which really didn't gain common usage until in to the '80's. We were and are a slow and arrogant lot. Potters knew about it for years. Without the flue there is too much back pressure from combustion and the door doesn't address it efficiently. It does somewhat but not well.
In actuality I saw my first recuperated small furnace back around 1978 which is about the time the private shops began to outstrip the universities in knowledge of studio operation. By 1985 the private shops were really the cutting edge on that stuff. By 1990 the private shops had outstripped the schools on design innovation as well in my opinion. I don't know when Tm Ash did his first ribbon burner. I ran my first homemade one in about 1995. I had stopped using the style from Dudley because they were a bit fragile. Somewhere in there, the thermjets got into the mix and have a big seat at the table. Premixing air and gas has been round for at least 25 years at the studio level.

Industry by the way has known most of this stuff for years. It's just that the things like the maxxon mini pre mix blower was just that, A tiny unit by industry standards to light the pilot. We tried to run furnaces with them and it worked.

Mitcheal Veenstra

11-04-2011, 05:49 PM

So it kind of started the way it did because that's the way the first guys did it and things were slow to change because we have a strong independent streak eh? :)

If you had a flue on a furnace to help with the back pressure, would you gain any benefit from more direct heating of the glass mass with a top mounted burner on a small furnace?

I have seen some of the smaller pot furnaces fired with a burner located along the upper portion of where the crucible is and fires in tangentially to attempt to avoid flame impingement on the pot. The flue being another port at the bottom exiting out the other way leading to the stack. I think it's to provide a cylindrical flame path for better burn. It seems that it should be the other way, fire from the bottom port and flue it from the top, but I don't know if it matters which way it's done.

I know that with a big enough burner just about any way you do it will work, I've seen a traveling demo furnace before built out of half of a 55 gallon drum, lined with frax with a shallow crucible, side fired venture burner above the glass line back in the mid to late 90's.. it worked, just isn't very efficient even for a demo. It used a lot of propane...

Pete VanderLaan

11-04-2011, 06:05 PM

As you said, it used a lot of propane. Putting the flue down below forces a long flame to develop which is more efficient. I still build side fire above glass line, flue down below on the burner side, stack seven feet tall.

I think the worst way to fire a pot is bottom fired. It just gets the crucible base too hot and it fails far sooner. Interestingly, when we do autopsy work on pots, cracks from thermal issues are usually on the opposite side of the pot from the actual insult. Pinking of the pot is a sign and crows feet cracks are indicative of thermal issues.

Even now, people build furnaces the way the shop had them that they learned in. Nick put those crystalite brick daytanks ( fancy name!) together for a song designed for a short run. They remained the institutional design for the next 20 years. Never a really good idea but it sure kicked things off right. Those crystalites were a nice brick for about a year, maybe a little longer. We replaced furnaces a lot in those days. It turned out that the primary use for crystalites was crematorium liners.

Patrick Casanova

11-04-2011, 07:15 PM

Peter,

I know you don't believe in the burner being below the pot... but I just got 14 years out of my old one which fired below the pot and I'm just about done with my new one and I'm doing it again. I based my original design off of Wes's in Henry's first book. With Wes being here in Wisc. I went over and looked at his and we talked about design improvements he'd do, what he never do again, etc.. He was just over last month and we looked over the carcass of the old one and talked about what I'm doing differently this time. I had 5 old pots sitting around the building and when turned over so you could not see the rim you could not tell where the burner was. The only way you can tell is by the discoloration on the lip from gathering.

I believe that if the chamber is designed correctly it should not be an issue. Potters all across the world fire their kilns with the burners low. Bringing "green ware" up to temp is more problematic than bringing up a vitrified clay body/pot. As you know I'm a believer in your pots and their ability to withstand thermal shock. I believe that I stress a pot more by charging too much cold batch at once and creating too great a temperature disparity between the hot outer wall and the now chilled inner surface. The temp difference is huge.

Anyhow, I'm ready to light up the new one and it has the burner below the pot. I'll keep you posted.

Pete VanderLaan

11-04-2011, 08:09 PM

My experience selling pots tells me you must have an excellent design because the failure rate on bottom fired pots is simply much higher than with side fired ones. I won't argue it any more than I will argue about investing them.

Patrick Casanova

11-04-2011, 09:34 PM

What were you describing when you referred to "Pinking of the pot is a sign"? I've got a Korundal XD that was used as a sill that developed a pink cast to it. I also used the same brick as the post for the pot and they remained white.

Mitcheal Veenstra

11-04-2011, 10:42 PM

Thanks Pete for the history lesson and advice. I try to learn as much as I can before I step off the cliff :)

Pete VanderLaan

11-05-2011, 05:42 AM

If you do bottom fire, the bag wall has to be complete and have no flame impingement on the pot. That takes a lot of room.

I can't say about the brick but high alumina pots take a pinking when the are getting too hot and cold repeatedly

Tom Fuhrman

11-05-2011, 08:47 AM

did anyone here ever see the cupola furnacedesign that Nick was working on? I was to his shop over 30years ago and he was working on a furnace that fired from 5 sides and had a pipe for mixing going all the way around the outside of the furnace. I never saw it run and at the time I was so "green" to most of this that I wasn't sure what he was talking about most of the time, as if you could ever know exactly what Nick was thinking.
Prior to 1974, energy usage and cost was never much of an issue so new designs were not investigated much.

Scott Novota

11-05-2011, 01:08 PM

I love this stuff. History and how everything happend. Book material I tell ya.

Mitcheal Veenstra

11-05-2011, 04:05 PM

I love this stuff. History and how everything happend. Book material I tell ya.

part of why I asked the question! ;)

So, for my own understanding..

If I go with a venturi burner system without a blower, on small scale, am I going to be using a lot more gas than I would with an equivalent system running with a cheap blower on it?

I would rather be able to run without electricity for the furnace/glory, but don't want to really shoot myself in the foot fuel wise.. I've got to have some power for a 110 kiln for an annealer, unless I figure something out about a gas fired annealer, but I don't EVEN know where to start there...

I know that the common approach now for small scale is to go electric, but this is kind of a different case where I don't have ready access to 240 volts for a lauckner style electric furnace. I built one of his little 15 pound color pot wonders back in 2000 or so. It was a great unit to learn out of for a while, but where I'm setting things up now I really don't have access to the power for something like that or his larger 40 pounder.

Dave Bross

11-06-2011, 08:07 AM

You could build a combo furnace/glory.

Basically a furnace with increased chamber size and a front port to reheat in.

That would use less gas than 2 units but upping the chamber size ups the BTUs needed to heat it too, so maybe not a big difference there.

Savings in only needing 1/2 the refractories would be big.

I know I'm bad about trying a reheat from too cold sometimes....fatal error if you're working over your pot of clear. Something to think about come design time.

I have something like this in my glory, with pots in the back melting color under a top hatch and room in the front for reheats, keeping me from popping too-cold reheats into the glass in the pot. Idiot proofing!

I have some old mobile home steps to get up high enough to access the top hatch for the pots in the back. All that "stair stepping" can get old in heavy production...but then I am a lazy-ass glassblower by nature so it's probably good for me. There are some pics of the glory and me working out of it on the cane pulling page on my website at www.davebross.com/sitemap.html.

Reheats will be slower with the "glory" running at glass holding/melting temp.

Look up some of the old posts on pipe burners too. I've got one somewhere detailing the build on a homemade venturi. Fun! Dangerous!

Tom Fuhrman

11-06-2011, 08:19 AM

most of the factories, up until recrent times never had glory holes. they just used the furnace for all their reheats, but I believe they ran their furnaces a bit hotter than most do now. conservation of fuel was not a consideration until recent times. The biggest expense was always the labor.

Pete VanderLaan

11-06-2011, 07:13 PM

I think my monthly gas bill in 1972 was around $90.00. Tom is right about worrying over the bill. None of the schools had individual meters for the glass shop so the administration never had a clue about what it actually cost.

Sure, you can use a venturi for the burner, just make sure it's big enough to supply the BTU's you think you need. If you do use one, do put a BASO valve on it so if it blows out, it doesn't keep pumping gas. I remember the ones I saw once at CE Refractories in St Louis on a huge beehive kiln. The stuff being fired was off loaded from a flatcar pushed in by a little yard engine. Forklifts came off with the stuff, then they bricked up the rail tracks, pulled out and bricked the door. The venturis were about 12 feet long and probably weighed about five hundred pounds each. They had garden hoses for gas lines. The kiln had either 12 or 16 burners. The burners were all crudely balanced on AZS slabs.

Josh Gillispie

07-22-2015, 09:54 AM

Patrick,

I'm extremely intrigued about your history of designing your burner below the crucible. Are you still firing this way? And what are your reasons for doing so, efficiency? Thanks.

Has anyone else had success with this? Or is the consensus that it is just too risky? Pete's caution carries a lot of weight, especially concerning flame impingement, but maybe its worth a bit of exploration, if there are economic benefits to firing this way.

Pete VanderLaan

07-22-2015, 10:10 AM

To be clear Josh, I spent a good deal of time investigating temperature differences from floor to crown in both side fired and bottom fired furnaces. The bottom fired consistently ran 4-500F hotter in the floor sections than side fired across the top of the pot. I did nt think that one could get that hot without some O2 assist but I have a very good optical pyrometer and it was consistent. I certainly am not aware of any economic benefit from doing it. Further, if your pot springs a leak, you'll have issues.

Your pot won't stand being run up above 2600F for long, Pat's experience not withstanding. If you do bottom fire, put the pot on a very high quality shelf and don't hit the pot with even a hint of flame.

Josh Gillispie

07-22-2015, 10:41 AM

Thanks again Pete,

Crystal clear. That info probably saved me $10,000 and two decades of banging my head against the wall.

Firing ceramics and glass are really two very different animals though we tend to think of it as just gas and heat. Heat circulation in a big kiln full of pots is not the same as around one crucible. All that said, the shortest flame length is the least efficient and that is what updraft kilns still use. The potters have been slow to change as well in many cases.

I completely agree with Pete on the top side fired design, but I put my exhaust at the same level as the burner. That is somewhat forced by the recuperator design. However, when you bottom vent, the door becomes the easiest path when open. I think my furnaces are way more comfortable to gather out of for that reason.

Pete thinks it is chancy to push a crucible over 80 melts, but since going side fired over the crucible, I have been getting 120 melts.

The rep of venturis being less "efficient" than blower driven is a misunderstanding of power vs efficiency. The amount of heat you can push through a burner is "power" and is dependent on the pressure of the air flow. Venturi burners do have lower pressure and therefore you need a larger burner head to push the comparable amount of gas and air. But, you can get the same heat and efficiency out of venturi burners. No combustion engineering references will disagree with that.

Way back when we all followed Nick, it was very easy to build a brick box and carve or cast a simple burner throat into the crown. You could do the whole thing in a day easily. We built our first ones out of recovered sugar mill fire box bricks. They lasted about three to four months. Crystalite was a great improvement at that time.

The Penland area furnace designs of the late 70s were inspired by contact with the Fenton factory I think. They got me going in a whole different direction. Round design around a single round crucible, but bottom fired came from ceramics. Not the best for glass.

Lots has changed!

Pete VanderLaan

07-22-2015, 04:06 PM

Bottom fired may actually be tied in to how easy it was to balance a blower on a rock compared to doing all that welding.

Years ago, I was at a huge refractory facility in St Louis called CE Refractories. They had a beehive kiln that you actually drove a flat car into and the flat car top was at grade in the kiln. So, one drove the product to be fired onto the flat car on Scorifiers and then the fork lift would drive off and park the greenware somewhere inside this beehive and then go back for more until the kiln was filled. Then, the entire floor was bricked up to grade and the sixteen venturi burners were turned on.

I have never seen such large burners in my life. They were about fifteen feet long and the gas lines were actually garden hoses. The thing that I liked about them best of all was that they were all precariously stacked on top of pieces of AZS- about 1,000 lbs of it per burner.

Now Hugh, I can get Charlie Correll completely animated over venturis vs blowers. Dudley wants me to take this little kiln and put a small regulator in my system to grab 25 PSI. My regular system is 2 lbs. This attracted me because I worry over power failures with this little color melter. Charlie got worked and told me it would be 15-20 percent more efficient if I used a blower. Now it's not like I'm just an old hard brick stacker. I want to be efficient but the two of you carved out this corner of the world. The thing will not be recuperated as will the redesign on the moly which is being switched over to gas.

I won't advocate going over 70 but that was based on only batch, getting to 2375F and maybe pushing 90 melts. I don't want to tell people it will run forever when it is getting turned on and off constantly which is really hard on pots.